Conventionally, a pressure sensor for detecting a pressure by using a semiconductor type diaphragm uses a gel material for protecting a pressure introduction portion in a pedestal portion. The pressure sensors disclosed in the following Japanese and U.S. patent documents JP-A-H4-370726, JP-A-H5-133827, JP-A-H11-201845 and U.S. Pat. No. 6,651,508 have a structure that uses the gel material for protection of the pedestal. The pressure sensor in the disclosure includes a semiconductor diaphragm sensor chip having an element of piezoresistance on one side and a concave portion on a reverse side of a sensor area, and a pedestal made of a glass or the like having a through hole that connects the concave portion to an outside of the sensor.
The pedestal is connected to the sensor chip by an anodic bonding, and the concave portion of the sensor chip is filled with a gel material for protecting of a reverse side of the diaphragm sensor chip.
The diaphragm sensor chip detects a pressure introduced from the through hole that has the gel material contained therein. The sensor chip is a so-called “reverse side sealed type diaphragm sensor chip.” The sensor chip outputs a signal that is proportional to an applied pressure based on the piezoresistance effect.
Protection of the sensor chip is required because the chip is used to detect a pressure of a liquid or a gas that has a corrosive effect. The gel material is used to protect the sensor chip from corrosion caused by the liquid or the gas.
The gel material is also effective for protecting breakage, distortion or the like of the diaphragm sensor chip caused by an expansion force of a freezing water in the through hole.
The pressure sensor is used to detect an exhaust gas pressure from a diesel engine or the like. More practically, the pressure sensor detects the exhaust gas pressure in or around a DPF (Diesel Particulate Filter) or a EGR (Exhaust Gas Re-circulation) system.
However, the pressure sensor is expected to be working in an exhaust pipe in a vehicle or the like where the temperature of the environment ranges widely from 0 degree to 100 degrees in Celsius. Therefore, the gel material in the through hole hardens at the temperature of, for example, −30 degrees, thereby yielding a distortion force for pressing the reverse side of the diaphragm. As a result, the diaphragm portion of the sensor chip is distorted to generate a false output. That is, the pressure sensor of a conventional type does not have a measure to handle the situation such as an extreme low temperature or the like.